Bright photoluminescence (PL) and small dimensions (typical diameter smaller than 20 nm) of semiconductor nanocrystals, also referred to as quantum dots (QDs), have led to a significant interest in this material as an attractive replacement for organic dye fluorescent probes used in biomolecular detection and cellular imaging [T. Jovin; “Quantum dots finally come of age”, Nature Biotechnol. 21, 32 (2003)]. Commercially available colloidal QDs, such as CdSe capped with ZnS and encased in biocompatible shells, emit in the wavelength range of 400-650 nm depending on the QD diameter. A cap layer applied to the QD will play the role of passivating the QD surface and reduce the concentration of non-radiative recombination centers, the latter being responsible for quenching the PL radiation. However, the intensity of the PL radiation decreases with increasing thickness of the cap layer. Thus, it is of practical interest to develop QD surface passivation methods using thin cap layers, ultimately made of monolayers of a specific material. Materials that, in addition to the QD surface passivating feature, could be used as anchors for conjugating targeted biomolecules are also of interest. This is currently a subject of investigation in many laboratories.
Current specimen preparation schemes utilize water-soluble nanocrystals, i.e. “free-standing” QDs. Generally speaking, quenching of the luminescence radiation in unpassivated semiconductor nanocrystals as well as the little-understood surface chemistry of these particles have been key issues that limited the quest for the QD-based biodiagnostics. An example of unwanted properties of free-standing QDs is their intermittent PL, known as ‘blinking’ [R. P. Bagwe et al.: “Bioconjugated Luminescent Nanoparticles for Biological Applications”, J. Disp. Sci. Technol. 24, 453 (2003)]. Free-standing QDs do not yield emission/fluorescence spectra sufficiently narrow to make them available for simultaneous recognition of many different biomolecules. Practically, with this approach, it would be difficult to work with more than 10 color-tagged different biomolecules. Also, the study of biological systems in-vivo would require QDs emitting in the wavelength range of 800-1100 nm, which corresponds to the minimal optical absorption of combined blood, tissue and water (biological “optical window”). Such materials have yet to be developed.
In summary, difficult-to-implement technologies of compound semiconductors, problems with controlling the size, shape and uniformity of nanocrystals needed for achieving high accuracy testing and bright luminescence from QD containing biological assays, as well as the not well understood chemistry of QD surfaces have hindered the progress in the area of colloidal QD-based biodetection.